Polypropylene bypass caps for PSU 'lytics

Hi all,

What is the rationale for bypassing large electrolytics with small polyprops in power supplies? Anybody have strong feelings about it one way or another? I know Gus has expressed some dislike for bypassing in audio circuits, but what about power circuits? I'd love to know whay you guys think. I'll know what I think when my wife tells me.


Thanks -

I like polypros in tube power supplys alot. One guitar amp has all polypros in the power supply and PET cathode bypass caps. People seem to like it.

Cool,

Thanks guys.

:thumb: :thumb:

[quote author="buttachunk"]also note-- good polyester caps sound just as good as polyprop for PSUs :thumb: good polyester caps sound 'softer' than polyprop which is extremely clean. the extremely cheap BC polyesters from digikey ($.09 each for some values) sound extremely good :thumb:[/quote]

if they're in the psu cleaning ripples and smoothing hash -- shouldn't you not hear them rather than hear them?

Yes, thats exactly what I (we) were talking about (hearing) and polyester bannanas too.

[quote author="buttachunk"] small bypass caps work similiarly for higher frequencies-- they are the PSU's buffer for being able to supply a 10,000 times power increase around 10khz within 1 microsecond. that transient, if not properly powered, will be slowed/slurred and/or distort.

for low frequency information, you want a deeper reservoir (bigger cap) to feed the bigger waves, and for high frequency information you want a faster cap to prevent slurring the faster waves.[/quote]

ok -- that makes alot of sense -- bigger caps for the lows and smaller ones for the hi frquencys.

so how can i calcualate the value i need to smooth out around 12khz?

[quote author="persona non grata"]so how can i calcualate the value i need to smooth out around 12khz?[/quote]You can't, without knowing the impedance. Suffice to way, higher impedance, lower cap value, (and the reverse) Higher frequency, lower cap value (and the reverse).

unless you need to target a specific frequency (and if you do, you likely have a problem somewhere else to fix!) then just overkill it.

Keith

There's one thing that nobody seems to have touched on here. The practice of bypassing power supply 'lytics with film caps originated with the audiophiles, their rationale being that 'lytics have some amount of series inductance, which can cause their impedance to rise slightly at higher frequencies. All caps have some amount of series inductance, but the audiophiles consider that of 'lytics to be high enough to be objectionable.

Of course, if you follow this line of reasoning fully, then you must also be making a parallel resonant circuit by shunting the inductance in the 'lytic with the film cap, and a parallel resonant circuit has a very high impedance at its resonant frequency, and you could end up with a power supply that "rings" at some high frequency.

But how much series inductance does a 'lytic really have? I don't know offhand. But if it's high enough to be objectionable at audio frequencies, then it's also high enough that bypassing with a film cap could cause it to be resonant in or near the audio band. Aye, there's the rub!

Here's some good reading about 'lytic characteristics:
http://www.chemi-con.com/u7002/characteristics.php
Hmm, they're saying that really big 'lytics can have series inductance as high as 34 NANOhenries. That's .034uH. About as much inductance as you'd get from a couple of turns of wire.

I would answer this question for myself with careful measurements (with test equipment), since all you're going to get on the Internet is opinions and conjecture (see above! :wink: ).

http://www.groupdiy.com/index.php?topic=4192

This might be good to read again

From Morgan Jones' Valve Amplifiers:

Thanks, Cayocosta!

:thumb:

Jakob E.